Part A. The cell membrane
Objective highlights:
Label the components of the cell membrane that are involved in passive transport mechanisms.
Define diffusion and identify physiological examples of diffusion.
Key concepts to memorize:
Structure of the cell membrane
Phospholipid bilayer as the basic barrier.
Embedded proteins facilitating transport (integral and peripheral proteins).
Channels (ion channels, aquaporins for water).
Carriers/transporters (facilitated diffusion).
Role of cholesterol in membrane fluidity.
Passive transport mechanisms active in diffusion through membranes:
Simple diffusion: movement of small, nonpolar molecules directly through the lipid bilayer down their concentration gradient.
Facilitated diffusion: movement of polar or charged species via carrier proteins or channels down their concentration gradient.
Osmosis (special case of diffusion for water across a semipermeable membrane).
Define diffusion:
Net movement of particles from regions of higher concentration to regions of lower concentration due to random thermal motion.
Physiological examples: O₂ and CO₂ exchange across respiratory membranes; diffusion of ions through open ion channels in neurons; diffusion of lipid-soluble hormones through cell membranes.
Difference between diffusion of a gas (e.g., O₂) and diffusion of an ion (e.g., Na⁺):
Gas diffusion often occurs directly through the lipid bilayer when molecules are small and nonpolar (e.g., O₂, CO₂).
Ion diffusion requires diffusion through membrane proteins (ion channels, transporters) because ions are charged and poorly permeable to the lipid bilayer; diffusion is along electrochemical gradients.
Examples of diffusion in physiology:
O₂ diffuses from alveolar air into blood (and CO₂ in the reverse direction).
Na⁺ (and other ions) diffuse through specific channels/pores in cell membranes, contributing to membrane potential and signaling.
Conceptual links:
Passive transport does not require metabolic energy (no ATP directly needed for the diffusion itself).
Membrane composition and presence of channels/pores determine the rate and selectivity of diffusion.
Part B. Diffusion
Experimental setup overview:
Obtain two agar plates: one with 1.5% agar and one with 4.5% agar.
Remove a 5–6 mm circular plug from the center of each plate.
Fill the hole with KMnO₄ crystals; seal and incubate for 60 minutes.
Measure the diameter of the diffusion field (in mm).
Replicate with class results to allow comparison.
Data table (structure shown in transcript):
1.5% agar: Final diameter of diffusion field (mm) = _
4.5% agar: Final diameter of diffusion field (mm) = _
How this demonstrates diffusion:
Diffusion is shown by the spread of KMnO₄ from the concentrated plug into the surrounding agar.
Permeability differences arise from agar concentration; larger pores (lower agar %; 1.5%) allow faster diffusion than smaller pores (higher agar %; 4.5%).
Key concept: membrane porosity and diffusion rate
Heterogeneity in pore size affects diffusion rate through gel matrices; this models how membrane permeability changes with structural traits.
Question prompt from transcript:
How does this experiment demonstrate diffusion? (Answer should connect to concentration gradient and dye spread through the medium.)
Part B extension question:
What would make real cell membranes have different permeability? (Possible factors: pore size/number, lipid composition, cholesterol content, temperature, presence of transport proteins, membrane fluidity.)
Part C. Osmosis
Experimental setup:
Prepare 4 dialysis tubing segments, each 10–15 cm long.
Seal one end with a non-weighted clamp (pre-sealed on some setups).
Fill each with exactly 10.0 mL of 30% NaCl solution.
Remove air; seal with a second, weighted clamp, leaving space for volume gain.
Submerge each bag in a beaker of water that fully covers the bag.
At 15-minute intervals, remove one bag, empty its contents, and measure the bag volume to the nearest 0.1 mL using a graduated cylinder.
Repeat for 30, 45, 60 minutes; results for class replication will be shared.
Data collection template (structure shown in transcript):
Time (minutes): 15 | 30 | 45 | 60
Final Volume (mL): | | |
Volume Gain (mL): | | |
Rate (mL/min): | | |
Concept: influence of time on osmosis
Osmosis depends on the concentration gradient (30% NaCl inside vs. lower NaCl outside in water).
The longer the time, the more water moves across the dialysis membrane until a smaller driving force remains.
Could this experiment also demonstrate the influence of concentration gradient size on the rate of osmosis?
Yes: larger gradients (larger difference in solute concentration between inside and outside) generally increase the driving force for water movement, increasing the rate until equilibrium.
Equilibrium concept in osmosis with dialysis tubing:
Salt molecules (NaCl) cannot pass through the dialysis tubing (semipermeable membrane) in this setup; therefore, the concentrations on each side of the tubing can never be equal.
A state of equilibrium for water movement can be achieved (volume stabilization) without equalizing solute concentrations across the membrane.
Part D. Hypertonic, hypotonic, and isotonic solutions
Lab activity: Connect simulation “Osmosis – tonicity in red blood cells.”
A study involved placing 1 mL of blood in solutions with different NaCl concentrations (mM) and measuring percent transmittance (a proxy for cell changes affecting light transmission).
Data table (structure shown in transcript):
[NaCl] in mM vs Transmittance (%): 0 → 100; 50 → 99; 100 → 98; 120 → 88; 140 → 32; 160 → 24; 180 → 18; 200 → 13; 220 → 10; 240 → 10.
Summary prompt:
Summarize results of the simulation: as external NaCl concentration increases, transmittance generally decreases, indicating changes in RBC optics due to osmotic effects (cell shrinkage/hemolysis changes in light scattering).
Diagnostic question from transcript:
In Part C, was the solution in the beakers hypertonic, hypotonic, or isotonic to the solution in the dialysis bags? Explain your answer.
Core interpretation:
The dialysis bag contains a very concentrated salt solution (30% NaCl). External beaker solutions with 0–240 mM NaCl are far less concentrated, thus hypotonic relative to the dialysis bag contents. Water would tend to move from outside the bag into the bag to try to balance osmolarity, causing an increase in bag volume (if membrane permeability to water allows). Therefore, the external solutions are hypotonic to the dialysis bag contents.
Part E. Filtration
Experimental setup:
Obtain ~30 mL of a solution containing charcoal, sugar, and starch.
Filter the solution through a piece of 5P filter paper and collect the filtrate.
Test both the original solution and the filtrate for sugar and starch as follows:
Tests:
Benedict's Test for sugar:
Procedure: Mix equal amounts of the substance with Benedict's solution and heat in a water bath for ~3 minutes.
Positive result: Color change to green, yellow, orange, or red indicates presence of simple sugars (monosaccharides and/or disaccharides).
Iodine Test for starch:
Procedure: Place a small amount of substance in a well and add iodine-potassium-iodide (IKI).
Positive result: Color change to purple, dark green, or black indicates presence of starch (a polysaccharide).
Concept: how filtration demonstrates separation by size and selective passage through a membrane.
Key questions to answer:
Why test both the original solution and the filtrate for sugar and starch?
To determine which solutes pass through the filter paper and which are retained; to confirm that filtration removed larger molecules (e.g., starch) while allowing small ones (e.g., sugar) to pass, depending on pore size.
Was charcoal present in the filtrate? Explain.
Likely no. Charcoal particles are large and will be retained by the filter paper.
Was sugar present in the filtrate? Explain.
Yes, sugars are small molecules and typically pass through filter paper; Benedict's test would be positive in the filtrate if sugar passed.
Was starch present in the filtrate? Explain.
Likely no, starch is too large to pass through the fine filter paper and would remain in the original solution.
Part F. Experimental design
Prompt:
Explain why the above experiments and activities demonstrate good experimental design.
Key criteria for good experimental design in this context:
Replication: multiple trials to ensure reliability of results (class replication in diffusion and osmosis experiments).
Controls: use of a controlled setup (e.g., maintaining similar temperatures, volumes, and concentrations except for the variable under study).
Standardization: consistent preparation of agar concentrations, dialysis bag sizes, dialysis solution volumes, and filtration membranes.
Clear measurement outcomes: predefined metrics such as diffusion field diameter, final volumes, rate of osmosis, transmittance values, Benedict’s/IKI results.
Timely data collection: recording time points and volumes at specified intervals to observe changes over time.
Interpretability: linking observed results to underlying mechanisms (diffusion through pore size, osmosis driven by gradients, filtration by size exclusion).
Ethical and practical considerations: preferring non-living systems to illustrate these concepts, reducing need for complex living models.
Part G. Application and conceptual reasoning
Osmosis and visible skin changes during beach vs. pool exposure:
Observation: fingers and toes wrinkling is less pronounced after ocean exposure than after pool exposure.
Explanation: Osmosis drives water movement across the skin based on external solute concentrations. Freshwater pools are relatively hypotonic to skin cells, promoting water influx and wrinkling. Ocean water is hypertonic (high salinity), drawing water out of skin cells and reducing wrinkling.
Gas exchange and disease in the lungs:
Diseases like emphysema, tuberculosis, and pneumonia reduce surface area for gas exchange.
Effect on diffusion: decreased surface area lowers the rate of diffusion of gases (e.g., O₂ into blood and CO₂ out of blood), impairing oxygen delivery and carbon dioxide removal.
Renal filtration and blood pressure:
The kidneys filter blood; substances are cleared into urine depending on glomerular filtration dynamics.
If blood pressure increases, what happens to filtration?
Filtration rate typically increases (assuming autoregulatory mechanisms do not fully compensate). Higher hydrostatic pressure in the glomerulus raises filtration rate, up to physiological limits.
Filtration of glucose vs. penicillin:
Glucose and penicillin are both subject to filtration at the glomerulus, but handling differs:
Glucose: small molecules that are freely filtered; most are reabsorbed in the proximal tubule; not all appear in urine under normal conditions. Presence of glucose in urine indicates high blood glucose (glycosuria).
Penicillin: relatively small molecule but is cleared by filtration and subsequent excretion in urine; its excretion depends on filtration and tubular handling, including secretion.
Answer to which will be filtered: Both glucose and penicillin will be filtered by the glomerulus; however, glucose is largely reabsorbed (unless plasma glucose is high), whereas penicillin is typically cleared more directly via filtration and excretion.
Summary of key equations and concepts to memorize
Diffusion (Fick’s Law, qualitative summary):
J = -D rac{dC}{dx}
Movement from high to low concentration, driven by a concentration gradient.
Osmosis (definition):
Movement of water across a semipermeable membrane from lower solute concentration to higher solute concentration.
Osmotic gradient drives water movement; membranes differ in permeability to solutes vs. water.
Rate of osmosis (derived from dialysis tubing setup):
ext{Rate} = rac{ ext{Volume Gain}}{ ext{Time}}
ightarrow ext{units: mL/min}
Filtration (general principle):
Movement of fluid through a membrane due to hydrostatic pressure difference; in physiology often described by simplified forms of Starling-like relations or Darcy’s law:
Q = L_p A
abla P (qualitative form; permeability times area times pressure difference)
Filtration testing and solute passage:
In filtration experiments, test both the original solution and filtrate for solutes; use Benedict’s test for sugars and iodine test for starch to determine what passed through the filter.
Tonicity and RBC response:
Hypertonic external solution causes cells to shrink; hypotonic external solution causes cells to swell; isotonic solutions cause no net water movement.
Notes for exam prep:
Be able to explain why diffusion rates differ between 1.5% and 4.5% agar and relate this to pore size/viscosity of the medium.
Be able to describe how dialysis tubing acts as a model membrane and why certain solutes pass while others do not.
Be able to interpret transmittance vs. NaCl concentration data in the RBC simulation and relate it to changes in cell volume and light scattering.
Be prepared to justify experimental design choices (controls, replication, standardization) and to discuss real-world relevance and limitations of these membrane movement experiments.